#include<bits/stdc++.h>
using namespace std;
string s;
struct info{
	int times,index;
	char c;
	bool operator<(info&a){
		return index< a.index;
	}
};
map<char,info>m;

int main(){
	cin>>s;
	for(int i=0;i<s.length();i++){
		if(!m.count(s[i])){
			m[s[i]]=info{1,i,s[i]};
		}
		else{
			m[s[i]].times++;
		}
	}
	vector<info>vi;
	for(auto&i:m){
		if(i.second.times==1){
			vi.push_back(m[i.first]);
			
		}
	}
	sort(vi.begin(),vi.end());
	if(vi.size()==0)cout<<"no";
	else cout<<vi[0].c;
}